refactor: Move process exit handlers from the trackerless-network to the node package

[juslesan]

Summary

Moved exit handlers from the trackerless-network to the node package. This ensures that the operator nodes will still gracefully leave the network if the process is exited. For browser nodes the graceful leaving on unload event is still kept in the trackerless-network. Alternatively the on unload event handler could be removed as well. If this is done the users must be made aware that they need to call NetworkNode#stop manually when a browser window is closed.

Could be worth the effort to update the sdk's README to make sure that users are aware that they need to call StreamrClient#destroy always when shutting down the process.